The present invention relates to agricultural harvesters and, more particularly, to axial flow combine harvesters having a threshing and separating unit with at least one generally longitudinally arranged rotor for advancing crop material along a generally helical path.
In conventional combine harvesters crop material is fed between a threshing cylinder and an associated threshing concave, which extend transversely to the direction of combine travel. Hereby the crop is threshed over a comparatively short distance. Axial flow machines use one or more longitudinally arranged rotors which are rotated in associated chambers which are partly constituted by threshing and separating concaves. The crop material is subjected to a much longer threshing and separating action than in a conventional machine and therefore, the efficiency of axial flow machines is greater because a higher degree of separation is reached and the grain losses are reduced.
The concaves are provided above a cleaning system and constitute the lower portion of the chamber. The top portion thereof comprises a curved cover which has at its inner surface a set of fins to guide the crop rearwardly along a spiral path to the end of the threshing and separating system.
The pitch angle of the fins has a direct influence on the time interval the crop remains inside the chamber and hence on the chances for the grains to be separated from the straw. A larger pitch angle will increase the rearward speed of the crop flow and the portion of the grains that is not separated through the concaves and deposited onto the field (rotor losses). A smaller pitch angle will reduce the rearward speed, increase the dwelling time and reduce the rotor losses. However it is obvious that the increased amount of material around the threshing rotor also increases the power requirements for the rotor itself. So, apparently the rotor losses and rotor power requirements contradict each other when it comes to deciding which the pitch angle should be: chosen for the fins on the cover.
Nevertheless, there is a need for a threshing and separating apparatus which is efficient with respect to both the rotor power and the rotor losses.
According to present invention there is provided a combine harvester comprising:
a main frame;
a threshing and separating arrangement mounted to said main frame and including:
a generally cylindrical chamber having a generally longitudinal axis and comprising a lower separating concave assembly and an upper guide cover provided with fins arranged to move harvested crop material along a helical path inside said chamber; and
a rotor assembly mounted for rotation in said chamber and comprising a generally cylindrical rotor body with a separating section associated with said separating concave assembly and having a circumscribed diameter D;
characterised in that said fins are arranged at a pitch angle xcex1 substantially equal to the angle following from the formula (tan xcex1)4xc3x97D=7.6 mm.
For a rotor having a diameter substantially equal to 432 mm the angle xcex1 may be substantially equal to 20xc2x0. For a diameter of 559 mm the angle may be, equal to 19xc2x0.